1. Field of the Invention
The present invention relates to a radio communication system, and, more particularly, to a CW (Continuous Wave morse telegrphy) radio communication system which has a pitch control function and demodulates a received signal using digital signal processing.
2. Description of the Related Art
Conventionally, CW radio communication system are designed to be able to adjust a pitch (frequency of a demodulated signal) desirable by an operator by varying a frequency of a oscillated signal of a BFO (Beat-Frequency Oscillator).
Also known are CW radio communication system which employ a narrow-band filter such as an audio peak filter (APF) when receiving a CW to attenuate signals having other frequencies than the target frequency in order to reduce interference or noise.
The CW radio communication system using a narrow-band filter are disclosed in, for example, Japanese Examined Utility Model Publication No. hei 3-26693 and Japanese Examined Patent Publication No. sho 63-50893.
The CW radio communication system disclosed in the Japanese Examined Utility Model Publication No. hei 3-26693 has a narrow-band filter located at the subsequent stage of an intermediate frequency amplifier (IF-AMP) and an intermediate frequency mixer (IF-MIX). This CW radio communication system has a function to alter the local oscillator frequency input to the IF-MIX or to shift the center frequency of the narrow-band filter so that the center frequency of the narrow-band filter coincides with the frequency of an intermediate frequency signal (IF signal).
The radio communication system disclosed in the Japanese Examined Patent Publication No. sho 63-50893 has a narrow-band filter provided at the output stage of a beat-frequency oscillator and changes the frequency of the beat-frequency oscillator (BFO frequency) and the center frequency of the narrow-band filter by equal amounts in the same direction, so that the frequency of a demodulated signal coincides with the center frequency of the narrow-band filter.
In the radio communication system disclosed in the Japanese Examined Utility Model Publication No. hei 3-26693, if the local oscillator frequency is changed, the frequency of the demodulated signal does not match with the center frequency of the narrow-band filter and a demodulated signal is not output. This requires the readjustment of the center frequency of the narrow-band filter.
As the center frequency of the narrow-band filter and the frequency of the IF signal are adjusted using a variable element, it is difficult to make the center frequency of the narrow-band filter coincide with the frequency of the IF signal in responsive to each other.
In the radio communication system disclosed in the Japanese Examined Patent Publication No. sho 63-50893, the center frequency of the narrow-band filter and the BFO frequency are changed by equal amounts in the same direction. Even if the BFO frequency is changed, therefore, the readjustment of the center frequency of the narrow-band filter is unnecessary. Since signal processing is all performed on the analog base, the BFO frequency cannot be tuned to the center frequency of the narrow-band filter accurately.
In either method, when the center frequency of the narrow-band filter is changed, the bandwidth is also changed. From the viewpoint of the precision in the responsive tuning, it is difficult to set the frequency too narrow and it is difficult to improve the selectivity.